The invention is based on a process for the evaporation of a polymer solution, wherein the polymer solution flows through a heated helix and a vapor separator connected thereto.
In the manufacture of polymers in solution, a solution is obtained after the reaction. While maintaining a high product quality, the polymer has to be isolated from this solution in such a way as to minimize the residual solvent content of the end product. This necessitates solving a number of partial problems, which often demand diametrically opposed measures.
The pure polymer has a softening point which is often well above the boiling point of the solvent. As the boiling point of the solution is close to the boiling point of the solvent up to high polymer concentrations because of the very large molecular weight difference between polymer and solvent, the solution viscosity during the process initially increases sharply and can reach values well above 1000 Pas, which results in a poor heat transfer, requires large mixing forces and can lead to the temperature falling short of the glass transition temperature. Only in the final phase of the process, when the proportion of polymer has a substantial influence on the vapor pressure of the solution, does the solution temperature rise sharply and exceed the softening point of the polymer, which becomes noticeable as a drop in the viscosity. Towards the end of the process, the convective evaporation process changes to a diffusion process. This phase demands a prolonged residence time, a large exchange area, thorough mixing of the viscous product and a high partial pressure gradient.
The majority of products are very heat-sensitive, i.e. from this point of view they require a low temperature level and short residence times.
There have so far been two basic processes for achieving the object:
1. The solution is heated during spray drying and then atomized into a drying tower or an agglomerating tube with the aid of stripping steam. The solvent is evaporated off in this process to give a powder which is substantially solvent-free but enriched with condensate. This normally requires afterdrying. The main disadvantage of this process is the high cost in terms of energy and equipment.
2. The evaporation process uses a combination of evaporator types connected in series. An example of a known combination is: falling-film evaporator (pre-evaporation to 20 wt. % of polymer)+film evaporator (main evaporation to 50 wt. % of polymer)+double-shaft screw machine (residual evaporation to 500 xcexcg/g of residual solvent).
The combination multiphase helix+double-shaft screw machine has frequently been used in recent times. The multiphase helix, which is described in greater detail in patents DE 1 667 051 and DE 1 921 045, is based essentially on a flash evaporation and a downstream evaporation in the indirectly heated helix. A vapor separator is added downstream of the helix. To avoid excessively high viscosities, which can result in clogging of the tube, the process is carried out under positive pressure, which raises the temperature level and hence lowers the viscosity to the point where the danger of clogging is averted. On the other hand, the residence time of the product in the flow tube is so short that no product damage occurs. In this way the starting solution can generally be concentrated to 70 to 90 wt. % in one pass. The molar proportion of solvent, even at high polymer concentrations near the end of the tube, is still large enough for convection and not diffusion to dominate the evaporation process at this point. The high degree of concentration relieves the downstream screw machine, resulting in a lowering of the residual solvent content of the end product. The main disadvantages of this process are the high investment costs for the screw machine, the wear on the rotating parts, the many seals, especially those of the vapor domes, their susceptibility to failure because of the rotating parts, and the handling of the heavy machine parts.
The object of the invention is, during the concentration of polymers, to reduce the residual solvent or monomer content at least to the limiting value of less than 1 wt. % allowed by law, and at the same time to replace the expensive process engineering applied hitherto with simpler methods which can be carried out with inexpensive equipment.
This object is achieved according to the invention by a process for the evaporation of a polymer solution, wherein the polymer solution, which in the initial composition contains at least 30 wt. % of solvents and monomers, preferably 50 to 70 wt. %, is passed at a high flow velocity through a heated helix and a heated vapor separator connected thereto, characterized in that the temperatures of the heating means in the helix and the separator are above the softening point of the polymer and in that the two-phase mixture of polymer melt and solvent vapors and monomer vapors, formed in the helix, is expanded into the heated vapor separator to an absolute pressure in the range 10 mbar to 800 mbar.